Field of the Invention
The present invention relates to testing helmets for ballistic performance.
Description of the Prior Art
Current testing methodologies for combat helmet “impact” performance focus on shell perforation, shell back-face deformation (transient maximum and post-impact residual), and center-of-mass accelerations of the helmet-head system. The perforation and backface deformation specifications relate to ballistic impact and the helmet-head acceleration specifications to low-velocity impact.
Performance specifications for helmets used for ballistic protection typically set requirements on the minimum V50 ballistic limits for the helmet shells and on the transient deformations of the inner-wall of the shell. Preliminary V50 testing of candidate shell materials in flat panel form is used as an economical method for assessing baseline material configuration performance. Limitations are imposed on the maximum permissible shell (backface) deformations during ballistic impact. Testing is done with clay-filled headforms or with an empty helmet interior using x-ray, photographic, or metal witness plate techniques to capture material deformation during the ballistic impact event. Limitations are also placed on the maximum acceleration induced on a test headform with a finished helmet assembly in place during low-velocity impact by a weight dropped from a specific height, and on shell damage experienced by low-velocity impact with specified impact energy.
The National Institute of Justice, under the US Department of Justice, has NIJ Standard 0106.01 for Ballistic Helmets that establishes performance requirements and test methods for helmets intended for protection against various small arms and rifle threats. These standards assess ballistic penetration and ballistic impact attenuation. Ballistic penetration tests are conducted at specified threat velocities to “verify” V0 requirements using a special headform with sagittal and coronal plane cut-outs containing perforation witness plates. The ballistic impact attenuation tests use an accelerometer affixed to the center-of-mass aligned with the trajectory of the ballistic threat, and measured peak accelerations upon impact are limited to 400 g maximum.
H. P. White Laboratories, Inc. have a testing protocol titled “Test Procedure for Bullet Resistant Helmets” that examines ballistic resistance to penetration by bulleted ammunition and resistance to backface deformations by non-perforating bullet impacts. This procedure uses a similar headform as that used in the NIJ 0106.01 standard. The tests are also V0 verification tests with a very limited number of test shots and failure defined by witness plate perforation. The backface deformation tests utilize the same headform but with the coronal and sagittal slots packed with clay (Plastilini Number 1) to register the transient shell inner-wall deformations. Maximum deformations are recorded, but requirements or limitations are left to be specified by the sponsoring entity.
Understanding and quantifying shell perforation mechanics and load, impulse, and energy transfers to the head are critical for achieving balanced and optimized performance from new helmet systems. While V50 testing is adequate for assessing perforation resistance, transient backface deformation measurements are not capable of quantifying the load, impulse, and energy transfers to the head, and these are the driving forces for brain injury and trauma.
Current state-of-the-art in brain injury/damage risk models use a combination of experimental data and computational analysis methods. Experimental measurements are made with surrogate models to quantify and characterize the loads and deformations imposed at helmet-head interfaces under impact conditions. The experimental results are used as inputs to computational models for predicting the resulting evolution of mechanical state within the skull-brain system. The accuracy of the computational modeling predictions depends on the accuracy of the experimental inputs and the constitutive properties used for the skull and brain components. These computational predictions of the mechanical state can then be used with an appropriate bio-medical risk model(s) to assess the probability of brain injury/damage. Existing clay-based helmet personnel protective equipment (PPE) testing protocols cannot provide the types of data necessary for quantitative assessment of the risk of brain injury/trauma under non-perforating ballistic impacts. More bio-fidelic instrumented headform and/or cadaver testing protocols can provide the needed data, but the testing can be very complex (technically and legally), and costly because of low throughput.